To fasten a cutting insert in a throwaway cutting tool for performing grooving work and cut-off work, generally the insert is held between an upper jaw and a lower jaw, respectively provided at an upper and a lower position of a slit formed in the front end portion of a holder, and the slit is tightened with a screw so as to fasten the insert with the pressure. According to Patent Literature (PTL) 1 for example, a V-shaped or wave-shaped recessed groove is formed on each of the upper and lower faces of the insert, and a V-shaped or wave-shaped projection that fits in the recessed groove is formed on each of the upper jaw and the lower jaw of the holder, so that the insert is attached to the holder with the projection fitted in the recessed groove. Such a configuration increases the binding force.
With the mentioned binding method, the screw is located at a central portion of the upper jaw with which the screw is to be engaged, and the screw is tightened so as to warp the upper jaw, and therefore the end portions of the upper jaw are bent upward with respect to the bottom portion corresponding to the central portion where the screw is tightened. However, the insert is attached to either end portion of the holder in the width direction thereof, and hence the insert is downwardly pressed by the upper jaw and also subjected to a force exerted outward in the width direction of the holder, when the screw is tightened. Accordingly, the insert may tilt outward in the width direction of the holder, or may even fall off out of the holder.
To prevent the insert from tilting outward, PTL 2 proposes a holder in which the terminal length of the slit is set such that the side on which the insert is attached becomes longer than the other end, in other words the terminal end of the slit is formed to be oblique such that the side on which the insert is attached is located on the rear side and the other end is located on the front side.